The goal for this period of support is to elucidate the actions and interactions of luteinizing hormone-releasing hormone (LHRH), neuropeptide Y (NPY), and gonadal steroids at the anterior pituitary (AP) to control gonadotrophs (GTS) and luteinizing hormone (LH) and follicle-stimulating hormone (FSH) secretion. Studies will be conducted in vivo using a model in which a neonatal or adult AP is allografted beneath the renal capsule of an adult hypophysectomized (HYPX)-gonadectomized (GDX) host hamster. Unanesthetized hosts will be administered LHRH and/or NPY by pulsatile infusion through a venous cannula. Some hosts will receive sc implants of Silastic elastomer capsules containing testosterone (T), 5alpha- dihydrotestosterone (DHT), or contain immunoreactive LH, FSH, or both hormones using immunocytochemistry (ICC); size of LH and FSH cells using morphometric techniques; mitotic activity of LH and FSH cells by injecting hosts with 5-bromo-2'deoxyuridine and using ICC; T and E receptor expression in GTS using ICC; ultrastructural features of LH cells using electron microscopy and ICC; concentration and affinity of graft LHRH receptors using ligand binding assays; LHalpha, LHbeta, and FSHbeta mRNA expression in LH and FSH cells using in situ hybridization and ICC in flip- flopped serial sections; relative levels of the subunit mRNAs in grafts using blot hybridization, autoradiography, laser densitometric analysis, and scintillation counting; and concentrations of LH, FSH, and their subunits in grafts and plasma using radioimmunoassays. These experiments are designed to show how LHRH, NPY, and gonadal steroids interact directly at the AP in vivo at a site distant from the immediate presence of the hypothalamus and independent of the secretion of endogenous gonadal factors. The results are expected to clarify our understanding of the roles these hormones play in postnatal development of GTS and in the control of LH and FSH secretion in adult male and female hamsters.